priming sugar amount for a 2.5 gallon batch in a 5 gallon keg
 

I have been working on plans for some small batches. My plan is to keg these batches and I'm trying to work out the details. My original plan was to rack to the keg and add double the priming sugar I normally use. Will 2x the regular amount of priming sugar work?

Woah, never heard of "keg conditioning" with priming sugar in place of force carbing. All that extra head space in the keg could be a problem!

I do this quite often. A couple of things:

Use less priming sugar per gallon than you would use in bottles.

Second, you can mix up the sterilized sugar solution and add to the empty sanitized keg, then just rack the beer into the keg.

Finally, vent the oxygen from the keg by doing a few co2/vent cycles. Then, add enough co2 to ensure the keg I'd sealed properly. Carbonate at normal temperature (to allow yeast to do their thing).

less sugar? I was thinking I would need more.

I do not have any evidence or knowledge to back this up. But I would think along the same lines as you....more sugar. I mean, the reason we use the amount of sugar we do is so the carbonation stays in the beer. I would think you need to create more carbonation to fill the 2.5 gallons of headspace and still get the carbonation you want IN the beer.

Luckily, with a keg, you can add more CO2, or bleed it off if its not right. Maybe go with your gut and report your findings

You got it. I was told by my lhbs that only 1/2 of the amount would be required. However, every time I've done this it is because I want to both bottle some of the batch and keg some of it. So, I mix up the sugar solution, sterilize it, add to the empty (sanitized) keg, rack to the keg, then bottle (under low pressure) from the keg. Then I let the rest carbonate in the keg. Since I was doing both, I did the sugar mixture as needed for bottling (and didn't reduce it). In terms of what was left in the keg (and extra head-space), it wasn't an issue as I ensured all the oxygen was removed from the keg (using the co2/vent/co2/vent/co2/vent). A week later, there was PLENTY of pressure in the keg from carbonation (and I vented some off).

As you mentioned, what is nice about the keg is that you can always add co2 or vent as required.